CT and MR Findings of Parotid Masses: Benign versus Malignant

PURPOSE: To determine the differential findings of benign and malignant parotid masses, as seen on CT and MRimaging. MATERIALS AND METHODS: The CT(24 cases of benign and 10 cases of malignant masses) and MR imaging(18cases of benign and 9 cases of malignant masses) findings of parotid gland masses confirmed by surgery orhistopathology were analyzed by two radiologists ; they focused on size, cystic change, the presence ofcalcification within the mass, density or signal intensity and margin, degree of contrast enhancement andhomogeneity, location and bilaterality, associated findings-including infiltration into surrounding structures andlymphadenopathy. RESULTS: In one of the 34 cases seen on CT, precontrast images were not available. In 15 of 23benign cases(65.2%), the density of the mass, as seen on pre-contrast enhanced CT scan, was lower than that ofmuscle. In ten malignant cases, density lower than that of muscle was noted in only two cases (20%). OnT2-weighted images, low signal intensity to fat was noted in five of nine cases(55.5%) of malignant lesion, but inno cases involving benign parotid masses. On CT scanning, an indistinct margin of the masses was observed in fiveof 24 benign cases(20.8%) and three of ten malignant cases(30%), but on MR imaging, this was seen in three of 18benign cases(16.7%) and 6 of 9 malignant cases(66.7%). On pre-contrast enhanced CT scan, 15 of 23 benign casesshowed homogenous density, but 12 of these 15 (80%) changed to inhomogenous on post-contrast enhanced CT scan.Among the 12, pleomorphic adenoma accounted for ten cases(83.3%). On CT scanning, infiltration into surroundingstructures including subcutaneous fat tissue was observed in three of 24 benign cases(12.5%) and four of tenmalignant cases(40.0%) ; and on MR imaging, in one of 18 benign cases(5.5%) and six of nine malignantcases(66.7%). CONCLUSION: If a mass of lower attenuation than that of muscle is seen on pre-contrast enhanced CTscan, or density patterns change from homogenous on pre-contrast CT to inhomogenous on post-contrast CT scan, themass may be benign. However, for the differential diagnosis of benign and malignant parotid masses, the margin ofthe mass is not helpful. Masses which on T2-weighted MR images show an indistinct margin, lower signal intensityto fat and infiltration into surrounding structure are more likely to be malignant. CT and MR findings relating tomass size, cystic change within mass, and lymphadenopathy are not, however, helpful for the differential diagnosisof benign and malignant parotid masses.

CT Findings in Differential Diagnosis of Benign and Malignant Parotid Tumors

PURPOSE: To evaluate CT findings which may help differentiate benign from malignant parotid tumors. MATERIALS AND METHODS: The CT findings of seventy-one cases with surgically-proven parotid tumors were retrospectively analysed for size, location, margin, internal density, adjacent tissue plane and lymphadenopathy. RESULTS: The margin of the mass was smooth and sharp in most benign tumors (89.5%), and irregular or indistinct in twelve which were malignant (75%, p<0.01). With regard to internal density, 70.2% of benign tumors were homogeneous (similar to muscle) and 81.3% of malignant tumors were heterogeneous (p<0.01). When analysing low density patterns within the mass, focal low densities in benign tumors (11/17) and diffuse or scattered multifocal low densities in those which were maligant (8/13) were frequently seen. Three malignant tumors invaded adjacent muscles, the parapharyngeal space, and bones, each in one case, and twelve malignant and one benign tumor infiltrated the adjacent fascia or subcutaneous fat layer. In five patients with a malignant tumor, obliteration by the mass of the fat plane between the mastoid tip and styloid process was noted, suggesting facial nerve invasion, while in three cases of malignancy, lymphadenopathy greater than 1cm was seen. CONCLUSION: In differentiating malignant and benign parotid tumors, the presence of irregular or indistinct margin of the mass, and invasion of adjacent structures, are important. Lymph node enlargement greater than 1cm and diffuse internal low densities, which may suggest necrosis or cystic change were also helpful in differential diagnosis.